1. Field of the Invention
The present invention relates to a close contact type sensor. More in details, the present invention relates to an MOS type sensor apparatus formed by using TFT above a glass substrate.
2. Description of the Related Art
In recent years, information apparatus such as personal computers have spread widely and there has been an increase in request of reading various information by a personal computer as electronic information. Therefore, a digital still camera as a substitute for a conventional silver film camera and a scanner as means for reading printed matter printed on paper, considerably attract attention.
In a digital still camera, there is used an area sensor in which pixels in an image sensor portion are arranged two-dimensionally. In a scanner or a copy machine, there is used a line sensor in which pixels in an image sensor portion are arranged one-dimensionally.
Scanners are generally classified grossly into three types in accordance with a reading system. That is, scanners can grossly be classified into three types of (1) sheet feed type, (2) flat bed type and (3) pen type (handy type). (1) Sheet feed type is a system of fixing an image sensor portion of a scanner and reading draft by moving the draft by sheet feeding. (2) Flat bed type is a system of fixing draft on glass and reading the draft by moving an image sensor portion below glass. (3) Pen type is a system of reading draft by moving an image sensor portion above the draft by an operator. In this way, a line sensor is frequently used in a scanner.
In the above-described three scanner types, optical systems used therefor are substantially determined. In a scanner of (2) flat bed type, there is frequently adopted a reduction type optical system for finely reading an image. A lens used for the reduction type optical system is provided with a long focal length and accordingly, a distance between an object for reading and an image sensor portion is lengthened and the apparatus is large-sized.
It is necessary to downsize the apparatus in the case of (1) sheet feed type or (3) pen type (handy type). Therefore, there is adopted an optical system complying therewith. That is, there is frequently adopted a close contact type optical system. According to a close type optical system, there is arranged a rod lens array between an image sensor and an object for reading. A rod lens array is bundled with a number of distributed refractive index type lenses in a rod like shape. A rod lens array focuses an image in a one-to-one relationship and a distance between an object for reading and an image sensor portion is shorter than that of a reduction optical system.
As an optical system aiming at further downsizing of the apparatus by further shortening a distance between an object for reading and an image sensor portion, there is provided a completely close contact type. This is an optical system for reading an object for reading by bringing the object and an image sensor portion substantially into close contact with each other without arranging a lens therebetween. A protective film, thin protective glass or an optical fiber plate is arranged between the object for reading and the image sensor portion. The optical fiber plate is constituted by bundling a number of optical fibers and slicing the bundle in a shape of a plate.
There is introduced the above-described classification of optical systems in “A close contact image sensor infiltrating facsimiles, expels a reduction type with easiness-to-use as an arm?” in Nikkei Electronics: Apr. 3, 1989 (No. 470): p. 159. There is introduced a rod lens array using a distributed reflective index type lens in “Mass production of a lens array made of plastic for close contact sensors of facsimiles” in Nikkei Electronics: Nov. 13, 1989 (No. 486): p. 122. There is introduced an example of a completely close contact type in “Development of a CdS—CdSe image sensor of a completely close contact type” in Nikkei Electronics: Feb. 19, 1990 (No. 493): p. 112. There is introduced an example of completely close contact type using an optical fiber plate in “Development of a completely close contact type image sensor of a multiple tips system” Nikkei Electronics: Sep. 14, 1992 (No. 563): p. 80.
As an image sensor element, there is frequently used a sensor of a CCD type or a single crystal CMOS type. FIG. 2 shows a sectional view in the case of adopting a close contact type optical system by using these elements. There is arranged an optical system 10002 such as a rod lens array above an image sensor 10001 of a CCD type (CMOS type). The optical system 10002 is used for projecting an image on a draft onto the image sensor 10001. The relationship between the image and the sensor is constituted by an equal magnification system. A light source 10003 is arranged at a position capable of irradiating light to a reading object 10004. LED or a fluorescent lamp is used as a kind of the light source used. Further, glass 10005 is arranged at the topmost portion. The reading object 10004 is arranged on the glass 10005. The operation is as follows. First, light emitted from the light source 10003 passes through the glass 10005 and is incident on draft. Further, the light is reflected by the reading object 10004, passes through the glass 10005 and is incident on the optical system 10002. The light incident on the optical system 10002 is incident on the image sensor 10001 and is photoelectrically converted at the image sensor 10001. Further, a signal converted into electricity is read to outside. After reading signals of one column by the image sensor, a scanner 10006 is moved and similar operation is repeated again.
As a constitution of using other image sensor element, there is provided a sensor formed with TFTs and photodiodes by using a-Si or p-Si above glass. FIG. 3 shows a sectional view of a line sensor when a completely close contact type optical system is adopted by using these elements. According to a completely close contact type, it is necessary to efficiently irradiate light to a reading object 304. Therefore, it is preferable that a substrate per se is transparent. Therefore, in a completely close contact type, not a single crystal substrate which does not transmit light but transparent glass is frequently used. In FIG. 3, a light receiving portion 306 is formed at glass 305 and a vicinity of the light receiving portion 306 is formed with an irradiation window 307 for transmitting light. Light emitted from a light source 303 is incident on a rear face of the glass 305, passes through the irradiation window 307, passes through an optical system 302 and is incident on a reading object 304. Light incident on the reading object 304, is reflected thereby, passes through the optical system 302 again and is incident on the light receiving portion 306. A light shielding window is frequently formed between the glass 305 and the light receiving portion 306 at portions other than the irradiation window 307 to prevent influence of light incident on the rear face of the glass 305 from being effected.
FIGS. 4A and 4B show views viewing from above a pixel of a sensor fabricated on glass. In FIG. 4A, a single piece of the irradiation window 307 is arranged at the center of one pixel of the light receiving portion 306. In FIG. 4B, a single piece of the irradiation window 307 is arranged contiguous to one pixel of the light receiving portion 306. These are published also in “A close contact image sensor infiltrating facsimiles, expel the reduction type with easiness-to-use as an arm?” in Nikkei Electronics: Apr. 3. 1989 (No. 470): p. 159. In this way, conventionally, a single piece of pixel is arranged with only a single piece of irradiation window.
FIGS. 5A and 5B show simple constitution views each for a single piece of pixel. In FIG. 5A, there is a single piece of the irradiation window 307 and there is the light receiving portion 306 for carrying out photoelectric conversion contiguous thereto. There is arranged therebelow, a circuit portion 502 of a switching transistor, a resetting transistor, am amplifying transistor and the like for resetting the light receiving portion 306 or amplifying a signal produced at the light receiving portion 306. The light receiving portion 306 and the circuit portion 502 in combination, is referred to as a sensor circuit portion. That is, a single piece of a pixel 501 is constituted by the sensor circuit portion and the irradiation window portion 307 and a plurality of the pixels 501 are arranged to thereby constitute a line sensor or an area sensor.
FIG. 5B is basically the same as FIG. 5A and is a constitution view when the light receiving portion 306 and the circuit portion 502 are arranged to overlap. It is necessary that the irradiation window 307 is transparent since light needs to transmit therethrough. Therefore, the irradiation window 307 and the circuit portion 502 are not arranged to overlap. Meanwhile, the light receiving portion 306 and the circuit portion 502 can be arranged to overlap since there is not such a restriction.
A description has been given of the case of using a line sensor. However, when a two-dimensional reading object is read by a line sensor, it is necessary to move the sensor or the reading object. Therefore, the apparatus is large-sized, reading speed is retarded or mechanical strength is weakened. Hence, researches have been carried out also on a close contact type area sensor arranged with pixels two-dimensionally. In order to make light incident on a reading object, a substrate needs to transmit light and therefore, the substrate needs to be transparent, for example, the substrate comprises glass. According to an area sensor, the pixels are arranged two-dimensionally and therefore, it is not necessary to move the area sensor in reading. Such a close contact type area sensor is published in “Amorphous Silicon Two-Dimensional Image Sensor and Its Application” Television Society Technical Report: Mar. 4, 1993: p. 25 or “Two-Dimensional Contact-Type Image Sensor Using Amorphous Silicon Photo-Transistor” Jpn. J. Appli. Phys. vol. 32 (1993) pp. 458-461.
Further, a description is given of a close contact type area sensor also in Japanese Patent Laid-Open No. 219823/1997 and there is published a view of a single pixel, that is, a view arranged with a single piece of irradiation window at a side of a light receiving portion. In this way, also in a close contact type area sensor, a single piece of pixel is arranged with only a single piece of irradiation window.
Next, a description will be given of a case of reading a reading object by color. When a color image is intended to read, a special method needs to use. Color formation methods are grossly classified into three types of (a) light source switching type, (b) filter switching type and (c) type for using color image sensor. According to (a) light source switching type, three colors of light sources (fluorescent lamp, LED etc.) are successively winked and image information of draft is successively read by monochromatic image sensors to thereby provide signal outputs of red, green and blue. According to (b) filter switching type, there are provided color filters of red, green and blue between a while color light source and monochromatic image sensors. Further, image information is successively read by switching the filters to thereby provide signal outputs of red, green and blue. According to (c) color image sensor type, color disintegration and reading are simultaneously carried out by a color image sensor integrated with three line image sensors and color filters in one package.
Next, a description will be given of a sensor portion for carrying out photoelectric conversion. Normally, light is converted into electricity by using a PN type photodiode. Otherwise, there is a PIN type diode, an avalanche type diode, an npn embedded type diode, a schottky type diode or a phototransistor. Other than these, there are a photoconductor for X-ray and a sensor for infrared ray. Concerning these, there is a description in “A Basis of a Solid Image Taking Element—a Mechanism of an Electronic Eye” written by Takao Ando, Hirohito Komobuchi: Nippon Ricoh Suppan Kai.
According to the conventional irradiation window 307, a single piece thereof is arranged for one pixel. Therefore, the light utilizing efficiency is not high, as a result, the signal is also weak. Further, since light irradiated also to portions other than the irradiation window 307, the light utilizing efficiency is not high and more power consumption is needed. Further, depending on a light source, light is not irradiated to a total of the irradiation window 307 and therefore, the light utilizing efficiency is not high and more power consumption is needed.
Here, in order to describe the light utilizing efficiency, firstly, a description will be given of Lambert's cosine law. Lambert's cosine law describes a reflection characteristic of light at a diffusing face. A diffusing face following Lambert's cosine law is referred to as a completely diffusing face and diffused light thereof is referred to as completely diffused light. Normal paper is near to a completely diffusing face and may approximately be regarded as a completely diffusing face with no problem.
Suppose that as shown by FIG. 6, incident light 601 is incident on a reflecting face 603 from an arbitrary direction. Then, when the reflecting face 603 is a completely diffusing face, the incident light is diffused and reflected in all of directions. A description will be given of an intensity of reflected light 602 at this occasion. First, an intensity of light reflected in a direction perpendicular to the reflecting face 603, that is, in a direction of a normal line or a perpendicular line, is designated by notation I0. And an angle made by the normal line of the reflecting face 603 and reflected light is defined as a reflection angle. An optical intensity I(θ) having a reflection angle of θ is given by I(θ)=I0 * cos θ. The optical intensity is not dependent on an angle of incidence of incident light. In this way, Lambert's cosine law states that the optical intensity of reflected light is the optical intensity I0 multiplied by cosine of the reflection angle.
Further, the light intensity described here is an intensity of light energy, that is, luminous intensity or luminous flux. When considered in term of brightness, in the case of complete diffusion, the brightness remains unchanged by an angle of viewing the reflecting face 603.
In this way, according to a completely diffusing face, regardless of an incident angle of incident light, reflected light is reflected in all of directions and reflected light in a direction of a normal line (perpendicular line) of the face is provided with the strongest optical intensity. Further, as the reflection angle is increased, the intensity of the reflected light is weakened. Normal paper may be regarded as a completely diffusing face as an approximation with excellent accuracy.
Based on Lambert's cosine law as mentioned above, a consideration will be given of the light utilizing efficiency of a case in which a single piece of irradiating window is arranged. Here, for simplicity, a consideration will be given of a case in which an optical system is not arranged. Even when an optical system is arranged, similar consideration can be given thereto.
Suppose that as shown by FIG. 7, there is a single piece of pixel formed with the light receiving portion 306 and the irradiation window 307 at the glass 305, above the reading object 304 constituting a completely diffusing face. Suppose that the irradiated light is irradiated from above. The irradiated light transmits through the irradiation window 307 and reaches the reading object 304.
First, when light is incident from the irradiation window 307 at a vicinity of the light receiving portion 306, reflected light from the reading object 304 is easy to be incident on the light receiving portion 306. Further, a reflection angle of the reflected light at the reading object 304 is small and therefore, an optical intensity thereof is strong as is known from Lambert's cosine law. That is, a large amount of light reflected by the reading object 304 is incident on the light receiving portion 306 and therefore, the light utilizing efficiency is high.
Meanwhile, when light is incident from the irradiation window 307 remote from the light receiving portion 306, reflected light from the reading object 304 hardly enters the light receiving portion and is transmitted again to the irradiation window 307. That is, the reflected light is wasted. Only light having a large reflection angle is incident on the light receiving portion. However, light having the large reflection light is provided with a small optical intensity as is known from Lambert's cosine law. Therefore, a large amount of light is not incident on the light receiving portion 306 and the light utilizing efficiency is low.
Next, a consideration will be given of a positional dependency of a light receiving rate of the light receiving portion 306.
First, reflected light from the reading object 304 is easy to be incident on the light receiving portion 306 at a vicinity of the irradiation window 307. Further, the optical intensity is also high since the reflection angle is small. That is, the light receiving rate is high at the light receiving portion 306 at a vicinity of the irradiation window 307.
Meanwhile, reflected light from the reading object 304 is difficult to be incident on the light receiving portion 306 remote from the irradiation window 307. Further, even when the reflected light is incident on the light receiving portion 306, the optical intensity is low since the reflection angle is large as is known from Lambert's cosine law. That is, the light receiving rate is low at the light receiving portion 306 remote from the irradiation window 307.
The above-described is summarized as follows. That is, even when single pieces of the large light receiving portion 306 and the large irradiation window 307 are arranged, light is utilized actually effectively only at a vicinity of a boundary between the light receiving portion 306 and the irradiation window 307. Therefore, even when the light receiving portion 306 is arranged at a location remote from the irradiation window 307, light is wasted. Further, even when the irradiation window 307 is arranged at a location remote from the light receiving portion 306, light is not utilized effectively. That is, when single pieces of the large light receiving portion 306 and the large irradiation window 307 are arranged, the light utilizing efficiency is very poor. When reflected light from the reading object 304 is not so much incident on the light receiving portion 306, a signal of the light receiving portion 306 is also weakened. As a result, a characteristic of the sensor such as sensitivity is deteriorated.
Next, a consideration will be given of a portion of light emitted from the light source 303 which is incident on the rear face of the glass 305 and transmits through the irradiation window 307. When light emitted from the light source 303 is irradiated to an entire face of the glass 305, light is irradiated also to a portion other than the irradiation window 307. A consideration will be given of the light utilizing efficiency in that case.
As shown by FIG. 7, light is irradiated to the irradiation window 307 from a side opposed to the reading object 304. Further, the irradiated light transmits through the irradiation window 307 and is irradiated to the reading object 304. The light is reflected by the reading object 304 and is incident on the light receiving portion 306. In the above-described procedure when light is irradiated to an entire face of the glass 305 from the side opposed to the reading object 304 in the direction of the irradiation window 307, light is irradiated also to the light receiving portion 306 and a sensor circuit portion such as other circuit portion (normally, a light shielding film is formed at portions other than the irradiation window 307, for example, between the light receiving portion 306 or the circuit portion and the glass 305 and only light reflected by the reading object 304 is incident on the light receiving portion 306). However, only light irradiated to the irradiation window 307 is actually utilized. That is, light irradiated to the sensor circuit portion is totally wasted. As a result, the light utilizing efficiency is lowered. Therefore, an increase in power consumption is caused for irradiating stronger light to the reading object 304.
Further, in the case in which light emitted from the light source 303 is irradiated only to a portion of the face, when positions of the region and the irradiation window 307 are shifted from each other, there is produced a region in which light is not incident on the irradiation window 307. That is, an amount of light which transmits through the irradiation window 307 and is irradiated to the reading object 304 is reduced. As a result, the light utilizing efficiency is lowered. Therefore, an increase in power consumption is caused for irradiating stronger light to the reading object 304.